P3D Ortho and TDC

[Michael Georgoff <Michael_Georgoff@xxxxxxxxxxxxxx>]

Seems to me that I can use averages to determine the ortho seat location.
Given American Stereo images taken with the Realist with 35mm lenses, and
expressed as a multiple of screen image height,  Greg Erker neatly suggests
"othro seat will be...  1.75 screen heights".  Bill Davis, SawZall in hand,
reckons "multiply onscreen image height by 1.5".  George Themelis advises
ortho is "... equal to the diagonal of the projected image".

Let imagine I want an image 45 inches tall.  We've got a film size of 23mm
high by 21mm wide.  Thus, if the image is 45 images tall, then it is 41
inches wide. George's diagonal works out to a bit over 60 inches, or about
1.3 times image height.

So, if ortho seat = x times image height, we've got George at 1.3, Bill at
1.5, and Greg at 1.75.  The average is 1.5166 (hell, we'll say Bill
"wins"...).

Therefore, if my image is 45 inches high,  that works out so that my nose
oughta be dead center and 67.5 inches away, about 5 1/2 feet.  Okay, the
ortho question is answered, no more stretch, no more squash.

But, what of my scenario as lonely projectionist?  If I want a 45 inch image
height, and I've got 5-inch lenses, then I'm 22.7 feet back (pass the
SawZall, Bill).  Ortho is way up there at 5.5 feet and I'm way back here at
22.7.  Does this mean my z-axis suffers 400 percent distortion, or is 4x
distorted?

Reworking the math so that the image is still 45 inches tall and the
projectionist gets see ortho at 5 1/5 feet, I calculate that my projection
lenses need to have about a 1.2 inch focal length...  Hmmm.. I don't think
this is gonna work, is it?

Maybe I'll do some reading about mirrors...  If I bounce light around
enough, I may be able to be an projectionist and still see ortho.  Sorta
like a gigantic Project-or-View...  Gee whiz, I assumed that the
projectionist could get ortho out of the deal somehow, kinda like a Kenner
Give-A-Show Projector, only more fun...

Thanks to all, on list and off, for info and education.


Michael Georgoff
Stretched Out, CA


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